Braking system for induction motors



PER CENT FULL LOAD TORQUE SINGLE PHASE BRAKING TORQUES AT VARIOUS SPEEDS FOR VARIOUS VALUES OF SECONDARY RESISTANCE x PER cam- SYNCHRONOUS SPEED H. L wlLcox BRAKING SYSTEM FOR INDUCTION MOTORS Filed Sept. 29, 1939 3 Sheets-Sheet 1 (I) Z 9 t (I) O l Fig.3 v

ls'o

INVENTOR.

HARRY L. WILCOX 6 H l S ATTORNEY.

March 4, 1941. L w cox 2,233,501

' BRAKING SYSTEI FOR INDUCTION MOTORS Filed Sept. 29, 1939 3 Sheets-Sheet 2 LOWER 4 HOIST 6 5 43 2 I 4 5 .9 INVENTOR. rl r2 r3 HARRY L. WILCOX Mamh .1941. .u. L. WILCOX 2,233,501

' amuse SYSTEM won mnuqw'xou MOTORS Filed Sept. 29, 1959 s Sheets-Sheet s Fig.6

INVENTOR.

HARRY L. WILCOX BY I Z a a 7 HIS I ATTORNEY.

Patented Mar. 4, 1941 lsosTA'rss OFFIQ BRAKING SYSTE FOR. INDUCTHQN Electric Controller 8..

Manufacturing Company,

Cleveland, Ohio, a corporation of Ohio Application September 18 Claims.

This application is a continuation in part of my copending application, Serial No. 225,611,

filed August 18, 1,938.

This invention relates to a braking system for electric motors, and particularly to a braking system for wound rotor induction motors.

Direct current motors have generally been used for the hoist motion of electrically operated heavy duty cranes because the speed of such motors is easily controlled during both lowering and hoisting of overhauling as well as nonoverhauling loads. Any tendency on the part of the load to drive a direct current motor faster than the speed for which it is adjusted is definitely opposed by the characteristics of the motor. For instance, if the load is overhauling, connections can easily be made which cause the motor, regardless of its speed at the instant, to become a generator and hold the overhauling load to within 5 or percent of a predetermined speed. A direct current motor can be made to operate at any definite speed within a wide speed range by merely adjusting the field strength, and for any preselected field strength operates at nearly constant speed regardless of the load. If speeds below the lowest speed obtainable by field control are desired. it is only necessary to insert some resistance in series with the armature or to provide an armature shunt or both.

"The polyphase wound rotor induction motor possesses certain advantages over the direct current motor. For example, since the induction motor has no commutator, it is of. course tree from commutation troubles, and the use of alternating current motors obviates the necessity of a motor generator set or other rectifier in an industrial plant supplied only with alternatlng current. However, because of the difiiculty of providing good speed control, induction motors have not been as widely adapted for driving the hoist motion of electrically operated heavy duty cranes, such as steel mill cnanes, as have direct current motors.

The no-load speed of an induction motor dewhich the motor is wound and the frequency of the source 0! supply, and the speed under loaded conditions depends principally upon the number of poles, the frequency of the source, and the resistance of the rotor circuit. Since the frequency is usually constant and the number of poles is not readily changeable, heretofore no satisfactory method has been available to control the speed of hoist motors when lowering 55 non-overhauling or overhauling loads. The most pends principally upon the number of poles for 29, 1939, Serial No. 297.175

common method of varying the speed of induction motors under loaded conditions has been to vary the secondary resistance. Increasing the secondary resistance when the motor is loaded causes a proportional reduction in speed, but as soon as the load is removed, the speed approaches the no-load or synchronous speed. If the loadis overhauling, the motor runs at speeds in excess of synchronous speed unless some means is provided to prevent such action. The insertion of extra resistance in the rotor circuit under overhauling load conditions at speeds'above synchronism results in greater excessive speeds. The usual means of speed control, therefore, are not satisfactory for controlling induction motors subject to overhauling and non-overhauling loads. The control equipment for the hoist motion of an electric crane of the steel mill type should be capable of providing a variety of different speeds in both directions with a minimum amount of effort and skill on the part of the operator, and the direction and speed should result from a logical movement of the manual control element. For example, such control apparatus should be capable of hoisting various sizes of loads slowly at first, then accelerating by controlled steps to full speed, lowering such loads short distances at slow speeds, lowering longer distances at high speed, and reducing the speed at will before striking the ground or other object. At all times during this sequence, and regardless of the speed of the motor at the instant, a braking means must be readily available, and each step in the sequence should take place as a result of a logical and simple movement of the manual control element, which is generally in the form of an operating lever for a drum or face plate type master switch. Moving the master switch handle further from the off position in either the hoisting or the lowering direction should always increase the speed, while moving it towards the off position should always cause a decrease in the speed regardless of whether the load is overhauling or not or whether the motor is operating above or below synchronous speed. With very little equipment, direct current motors can be made to respond uniformly and logically to the movements of a lever type master switch. Heretofore, various attempts have been made to obtain similar response from wound rotor induction motors.

Proper control of induction motors for the hoisting operation is relatively simple and in most controllers the methods used are substantially similar. The operation in the lowering direction, however, has presented great diiflculty. If the load is overhauling and the master switch operated so that downward power is applied to the motor with all of the secondary resistance in the rotor circuit, the motor quickly exceeds synchronous speed, the ultimate speed depending upon the magnitude of the load. If the master switch is then moved to positions which would give higher speeds with non-overhauling loads, the motor if subjected to an overhauling load, slows dowrirom its super-synchronous speed towards synchronous speed. This response is not desirable since the' operator must take into consideration the size of the load before determining which way to move the master switch to give the desired speed control.

One control system commonly employed to provide improved lowering control is known as counter-torque. system the master switch is first moved to the last position in the lowering direction, no circuits being made or interrupted by the' master switch until the last position is reached. At that time downward power is applied and the accelerating contactors'are closed in rapid succession to quickly shortcircuit all of the secondary resistance. When the master switch is moved back to the next to thelast position, lowering power is removed and hoisting power applied with all of the accelerating contactors open, resulting in a weak hoisting or countertorque which tends to decrease the lowering speed. Moving the master switch further toward the on position closes additional accelerating cont-actors to increase the counter-torque and i'urther decrease or stop the lowering. At

' standstill it is necessary for the operator to move the master switch quickly to the of! position or the counter-torque might-cause hoisting oi' the load. It ,is obvious that the movements of the load do not logically follow movements of the master switch and that the response of the motor depends largely upon the skill oi the operator. Another method which has been used for controlling lowering speeds of alternating current cranes makes use of the generator action present when direct current is supplied to the primary winding of an induction motor-.when the rotor is revolving. The necessity of a source of direct current is an obvious disadvantage of such'systems. It. is an object of this invention I to provide a simple speed control means for a polvphase induction motor which is effective throughout a range of speeds from standstill up to and including synchronous speeds and including over synchronous speeds.

It is a further object of this invention to Provide a control system for an alternating current crane hoist motor which permits definite speed control both below and above synchronous speed and in both the lowering and the hoisting directions regardless of whether the load is overhauling or not.

A further obiect is to provide a braking torque for an alternating current wound rotor induction motor without resor ing to direct current excitation or plugging connections and which is eifective throughout the normal speed range both above and below synchronous speeds.

A still further object is to provide a control system for a polyphase induction motor subject to both overhauling and non-overhauling loads which permits the motor to respond logically to movements of the manual control element re- Generally, in lowering by this gardless of the load and the instantaneous speed.

In accordance with this invention, a braking torque is produced by connecting together two terminals of the primary winding of a thrcephase wound rotor induction motor having a star connected primary winding and connecting the third terminal and the interconnected terminals to one phase of the three-phase source of supply. The single phase excitation sets up a stationary magnetic field. Current is induced in the revolving secondary winding due to its passing through the stationary field. This current is partially dissipated in the secondary windings and in the external secondary resistance, and, by virtue of transformer action, the induced current in the secondary also induces a circulating current in the closed circuit including the two interconnected legs of the primary winding. This circulating current produces a field which opposes rotation of the rotor. A braking torque is thereby produced and its value at speeds both above and below synchronism and at synchronism may 'be .controlled by regulating the secondary resistance.

By means of similar connections, three phase de1ta wound motors can be braked by single phase energization. Likewise, six phase motors can be braked by connecting them either as three-phase or double three-phase motors. Braking torques can be similarly produced in using two phase machines by-short circuiting one primary phase winding and exciting the other with single phase current. Thus, in each instance, the single phase braking is accomplished by short circuiting a portion or the primary winding and concurrently energizing all or a portion of the primary winding with single phase current.

Another object of this invention is to provide a control system for a polyphase induction motor in which the speed of the motor throughout its normal speed range both above and below synchronism is controlled by the application of single phase power to the primary winding.

A further object of this invention is to provide a means responsive to the electrical condition of the secondary winding of a wound rotor induction motor to insure proper operating speeds regardless of the size 01' the load.

Other objects and advantages will become apparent from the following specification, wherein reference is made to the drawings, in which:

Fig: 1 is a diagrammatic illustration of the control system of the present invention;

Fig. 2 is a'diagram indicating the position of the contactors for the various master switch positions during a hoisting operation;

Fig. 3 is a similar diagram indicating the position of the contactors during a lowering operation;

Fig. 4 is a wiring diagram showing the application of a. method of automatic operation of the system disclosed in Fig. 1 and the switching sequence indicated in Figs. 2 and 3; and

Fig. 5 is a graph showing the relation between braking torque and speed obtainable by this invention for various values of rotor resistance both below and above synchronous speeds.

Fig. 6 is a wiring diagram showing a portion of a. modified master switch and certain associ ated elements which can be substituted for the corresponding portion of the master switch of Fig. 4.

In Fig. 1, for simplicity of explanation, the contactors shown in Fig. 4 are illustrated as knife assessor switches, and similar elements in Flos- 1 and 4 are referred to loy means or the some reference numerals. Three conductors Ll, L2 sud L3 lndlcste a source of alternating current supply lfor a three-phase wound rotor induction motor l4 having a star connected primary winding l8 and a secondary winding l9. The contactor l8 when closed connects the conductor Ll to one termlnol of the primary windlrlg it end the conductor L2 to one side of o coutoctor ll. 'llme coutoctor ll when closed connects the conductor to one terminal of the primary tvluollus l8 and also connects the conductor to the terrclml of the primary winding to ll the corltector 66 is the closed position. sud H are closed, the motor l l rotstss so to hoist o, loool l6 (Flag. (l) suspended lfrom c cs ole ll wound on e drum as.

ll contsctor l2 when closed concurrently wltll the cootoctor lll connects the conductors cud to primary wludtog l8 to so orgcoslfrcv manner than when the coutector 0G closed, thereby causing the motor t' l to rotote so to lower the loed fill.

llle co'stsctor 963 when. closed the rlrl= mory connections for -sluole=plroce cl motor by connecting termlmls of the prl mory winding 68 together and orlc oi? the contacts cl each of the corl'tcolorc ll 92 to connect the interconnected the conductor 82 through coutcctor 06. Too third. terrrrlusl of the primary if is (ii-fill nested to the conductor LG 'oy mesons of the stall" tsctor M3 to com olete the lor courted-low line secondary to of the ooductlcn motor M Zoe either delto or ml ls sllovre comrectecl by m ses clip c. stow-connected lore'oclles r Esch the clrree re= sistorlce branches rt, r2 ecu reslsterloe sections, or, o, c co select to. A contector 2t when closed c'lrcults ill s ct'lora o, coutector 22 when closed short the sections c l, e. coutsctor circuits n fi m short circuits sectors o, lo creel c, cud coutsctor 23 when closed short clccults the sections a, c and d. I

Although the lllustrotlve e ocllmerlt primary winding is shown so the s or coal the seconclury winding es the rotor, lt olvclous from a study of the following descrlptlorl, that prlmomr winding were therotciluc tt lurllrlc? and the secondary winding were stationery, stroller operation would result.

In order to provide automatic supervlslou of the speed of the motor to e relay clrcult 3, comprising a condenser 3! connected in series wlth an operating winding 313w of c control relay l3 (Fig. 4), is connected across two of the slip-rlugs 253. At any particular s the frequency of the secondary current of an induction motor has a definite value, being equal to llne trequcncy st standstill and zero at synchronous speed. The induced voltage likewise varies from a maxlmum at standstill to zero at synchronous speed. Above synchronous speed regeneration occurs, the voltage and frequency of the induced secondary current increasing in proportion to the speed, the frequency becoming again equal to line fre-' quency at 200 percent synchronous speed. Since the decreasing frequency tends to Increase the relay currentand the decreasing voltage tends to decrease the relayfurrent. resulting in a. substantially constant current condition, an ordinary inductive relay connected across the slip-rings when the corltoctors l8 does not give a proper response. likewise, above synchronous speed the increasing frequency and increasing voltage result in s. relotlvely constant current condition in any mductlve circuit conuected to the rotor circuit.

The relay clrcult 38 ls fully described in Patcut No. 2,165,494 lsued to John lit. Leltch on July ll, 1939; It is sufficient here point out that the circuit 30 is responsive-to operate the relay $33 in accordance wlth the electrical coudltlon of the rotor winding clue to the phenomenon of series resonance whlcll comes c dlstluct drop in the relay current at e, predetermined secondary frequency, depending upon. cececltcnce of the condenser ll end the inductance the windlug 8320. The oddltlon of the condo ear to the circuit metres the rclsy clrcult ccpecltl've certolu frequencies and removes the to erroy to word constant current inherent lrr c purely tooluctlve circuit. The electrlcol connection of the Ell relcy circuit ill: to the sllp rings is controlled lay e. coutuctor Figs. 2 end 3, the solid dots indicate the con teeters wlolclr ere closed in response to movement of e. rooster switch to the vorlous positions, wlllle the clrcles lmdlcste the corltoctors which close lo response to oroerotlou or the relay 33.

when lt ls cleslred to holst the load the co ctors ltl and M be closed to supply tlu'ec phcse power to the prlmory winding of the motor fi l by moving e. rooster control element orucester swltcll fill (l lg. 4) to the first position l: the loolstlug d'lrcctlou. At this time the sec t We c3, lo, cea'lcl rt 02 cools of the rcslsterlce cl, 9% and r8 ore eficctlve lo. the seccutlery circuit end the motor exerts e. smell torque in. hoisting direction. In the second llolstlug the coutsctor 2t ls closed (l lg. all to short clrcult resistance sections 03, cellslug" lrlcrcsse lo the hoisting torque of the motor ,l l. the load; 56 stcrtecl to more lo. the first position, lts speed is increased, in

second oosltlorl. lo tlrc third position holsttlre corltector ls closed to short circuit both roslstcrlm sections c and "o, resultms m as further 'seuucrlce so for described is stscdsrd practice cool no claim of may novelty so to lt is herein In the lowering direction. however, merely selcctl ely shom-clrcultlus predetermined emouuts of the resistances M, 2*? and T3 does not give proper operation. However, the novel switching sequence now to be described, reference being made to Figs. l ond3, provides proper speed control in response to logical movements of the controller handle.

In the first position lowering the contoctors l0, l3 and 22 are closed. If e. non-overhauling load ls-to be lowered, no movement occurs since the motor I4 is energized only by single-phase current and can exert no torque.- However, if the load N is large enough to overcome the static lrlction of the motor 14 and the accompanying searing and shaftlng, the load l6 moves downwurd. If it were not for the connection made by the contactor. l3 and the single-phase excitation,

. the motor ll would accelerate to a. speed above closing of the secondary or rotor circuit through a predetermined resistance by the contactor 22, the kinetic energy of the rotor of the motor I4 is translated into electrical energy which additionally energizes the short circuited portion of the primary winding in a manner to oppose rotation of the rotor due to generator action. The flow of current in the short circuited portion of the primary winding It causes a corresponding increase in the secondary or rotor current. The kinetic energy of the rotor of the motor I4 is thus dissipated in the short circuited primary windings and in the secondary circuit including the external resistance sections 0 and d. A braking torque is thereby produced which limits the speed of the motor I to a safe value. With certain amounts or resistance in the secondary circuit the braking torque increases almost directly with the motor speed until speeds greatly in excess of synchronism are reached, as shown by curves VI, V2 and V3 0! Fig. 5. When a certain speed greatly in excess of synchronism is reached, the speed-torque curve for any particular value cf secondary resistance will reach a maximum and then decline as the speed increases further. The lower the secondary resistance the lower the speed at which these maxima occur. In Fig. 5, the curves VI and V6 represent speed-torque characteristics for various values 01' total secondary resistance which are related in magnitude as indicated.

There is a certain value of total secondary circuit resistance for each motor which gives the highest average braking torque and the best speed torque characteristic during braking, and increasing or decreasing the total secondary resistance above or below this optimum value either reduces the average braking torque or gives a speed torque characteristic which is not as desirable throughout, its entire range. This is clear from the curves oi. Fig. 5. In the case of one of the motors from, which the iii to plot these curves was obtained, an cartel: conciary re sistance of 5.7 times the int mist nee gave ml, torque h gl' 'r or Elli),

speed for overhauling loads. If the motor is lowering a load equal to full load of the motor, the speed increases to approximately 80% of synchronous speed.

Thus a speed change from 60% to 80% oi synchronous speed results from an increase in rotor resistance. Ii lighter overhauling loads are being lowered, the speed is less, but the increases are synchronous speed, for example, 50% of synchronou speed, or if the speed is already above the predetermined percentage of synchronous speed, the contactor 24 closes in response to operation of the frequency relay 13. The closure of the contactor 24 short circuits all of the resistances rl, T2, and r1 and prevents too great a speed 01 the motor H, the speed with the contactor 24 closed for overhauling loads being slightly above synchronous speed, and for nonoverhauling loads slightly below synchronous speed.

By moving the master switch repeatedly from the oil. position to the first position lowering, the average speed or heavy overhauling loads may be readily maintained at an extremely low value, by moving the master switch from the first position to the second position, a higher average speed may be maintained, and by moving the master switch from the second position to the third position, a still higher average lowering speed for overinau'iing loads may be obtained "d non-overhauling loads may be mover. wardly slowly.

and if l to accelerate which the re move the short circuit from the resistance sections c and d. In the sixth position lowering the contactor 22 is opened, removing the short circuit from the resistance section b. 11', while the master switch is in the fifth or sixth position, the motor should accelerate to a predetermined maximum supersynchronous speed, for example, 150% of synchronous speed, the relay 33 operates to close contactor 23, or contactors 22 andthe average braking torque from that speed down.

to zero is proportional to the area between the curve V3 and the abscissa of zero torque. It the master switch is returned to the second position instead of the first position a lesser value of braking torque is available, and its average value 'is the area between the curve VI and the abscissa of zero torque. Fig. 5 illustrates how diflerent values of secondary resistance may be chosen to give a desired braking characteristic throughout all normal speed ranges.

From the foregoing description it is to be noted that heavy overhauling loads are automatically prevented from moving downward too rapidly by means of the frequency relay 33 and thatthe speed changes in response to controller move-- ments are identical with those which occur with similar controller movements in the hoisting direction. That is, movement or the master switch away from the oflf position causes increases in speed regardless of load, and movement of the master switch toward the oil position causes decreases in speed.

In Fig. 4, the contactor i0 is shown as electromagnetically operable by means of a winding iflw. The contactors H and I! have operating windings llw and 12w and'normally closed auxiliary contacts Ho and Ma, respectively. The contactor l3 has an operating winding 13m and normally open auxiliary contacts "a. The contactor 24 has an operating winding 24w and the contactors H, 22 and 23 have operating windings 2lw, 22w and 23w, instantaneously responsive normally open auxiliary contacts 2la, 2m and 23a, and time delay, normally-openauxiliary contacts 21b, 22b and 23b, respectively. The re tardation of each of the contacts ilb, 22b and 23b may be obtained by any suitable means illusstrated by respective dash-pots Md, 22d and 13d.

The master switch 40 comprises a group of hoisting contact segments 42 to 41 and a group of lowering contact segments 48 to 31 and a common contact segment |i., A group of circuit terminal contacts 58 maybe selectively moved to the various positions I to I in the hoisting direction to contact the various segments ll to 41 or to the various positions i to 8 in the lowering direction to contact the various segments ll and 48 to 51.

The frequency relay circuit 30, comprising the condenser 3i and the operating winding 33w, controls the operation of the relay 33 which has normally open contacts 33d and 33b and normally closed contacts 33c. The contactor 3!.

which controls the connection of the relay circuit 3,0 to the rotor circuit, has an operating winding 32w.

A spring applied brake 31 is releasable upon energization of its operating winding 31w, which is connectible through a contactor 36, having an operating winding 36w, to the direct current terminals of a full-wave rectifier shown diagrammatically at 35. The brake 37 may be of any type which is released upon energization of an electrical operating means and is hereinafter referred to as a mechanical brake to distinguish it from the electrical braking action of the motor itself. The alternating current terminals of the rectifier 35 are connected to the conductors L2 and L3 by means of conductors S0 and Cl.

The operation of the control system shown in Fig. 4 in the hoisting direction will now be described..-

Movement of the master switch 40 to the first position in the hoisting direction causes energization of the operating windings low, llw and 36w of the contactors l0, ll and 36, respectively. The operating windings low and 36w are connected in parallel and the circuits there:- through are from the conductor l2 through a conductor 62, acircuit terminal 58 of the master switch 40, the segment 4|, the segment 42, another of the circuit terminals 58, a conductor 85, a conductor 63, the winding 36w, a conductor 34 to the conductor L3, and from the conductor 65 to the winding Him and a conductor 66 to the conductor L3. The energizing circuit for the winding llw is from the energized segment to the segment 43, and through a conductor 61,

the winding Hw, a conductor 68 and the con-- ductor 64 to the conductor L3. The contactors l0 and II in response to the energization of their operating windings lllw and 10 close their contacts to supply three-phase power to the primary winding l8 of the motor 54 which thereupon exerts torque in the hoisting direction. The closure of the contactor i0 supplies alternating current to the rectifier 35 over the conductors 6d and H, and the closure of the contactor 36, in response to the energization of its operating winding 38w, completes a direct current circuit tothe winding 3110 which causes release of the brake 31.

To increase the speed of the hoisting operation or to increase the hoisting torque, it the torque exerted in the first position is insufficient to move the load, the master switch 40 may be moved to the second position in the hoisting direction. When the master switch is in the second position, the winding zlw of the accelerating contactor ii is energized over a circuit from the energized segment I to the segment 41 and through conductors 69 and 10, the winding Ilw and a. conductor H to the conductor Ll. The accelerating contactor II, in response to energization of its operating winding 21w, closes its main contacts to short circuit the resistance sections a o! the resistance branches rl, r2, and r3.

Alter a time delay sumcient to permit closure of the contacts Ilb, movement of the master switch 40 to the third position in the hoisting direction causes the energization of the operating winding 22w or the accelerating contactor 22' over a circuit extending from the energized segment ll to the segment 43 and through a conductor II, the now closed contacts Ilb or the coning 2210 and the conductor H to the conductor Ll. The contactor 22 in response to energization of its operating winding 22w closes its main contacts to short-circuit the section b in addition to the section a of the resistance branches rl, r2, and rl.

After a time delay, a further increase in speed is obtainable by moving the master switch to the fourth position: in the hoisting direction thereby causing energization of the operating winding 2310 01' the accelerating contactor 23 over a circuit from the energized segment 4| to the segment and through a conductor 14, the now closed contacts 22b of the contactor II, a conductor 16, the winding 23w, and the conductor H to the conductor Ll. The contactor 28 in response to the energization of its operating winding 23w closes its main contacts to short circuit resistance sections a, b and c of the resistance branches rl, 1-2, and rl.

Energization oi the operating winding 24w 0! the accelerating contactor 24 is caused when the master switch 4. is moved to the arm position in the hoisting direction over a circuit from the energized segment 4| to the segment 44 and through a conductor 11, the now closed contacts 23b of the contactor 23, a conductor 18, the winding 2410, and the conductor II to the conductor Ll. The contactor 24, in response to the energization of its operating winding 23w, closes its contacts to short circuit all of the resistance in the three branches rl, 1'2 and r3.

It is apparent that the accelerating contactors 22, 23 and 24 close in response to the energize.- tion or their operating windings 1210, Nw and 2410, after time delay intervals due to the time delay action of the contacts Zlb, 22b and 23b occasioned by the dash-pots 2id, 22d and 23d. If the master switch 40 were moved quickly to the fifth position in the hoisting direction, the time delay action provides a proper acceleration rate for the induction motor l4 in a manner well understood.

The operation of the control system shown in Pig. 4 in the lowering direction will now be de-' scribed.

When the master switch 40 is moved to the first position in the lowering direction, the contactors H, II, 22 and 36 close, due to energization or their respective operating windings Him,

i810, 2210 and 3410. The energizing circuit for the operating winding llw of the contactor l0 is from the energized segment 4| to the segment 44 and through the conductor 65, the winding "In and the conductor 84 to the conductor L3. The energizing circuit for the winding "w 01' the contactor 36 is traceable from the energized conductor ll through the conductor 83, the winding "in, and the conductor 64 to the conductor L3 The energizing circuit for the operating'winding I310 01' the contactor ll is from the energized segment 4| to the segment I! and through a conductor III, the normally closed contacts I211 of the contactor It, a conductor Illl, the winding I810, a conductor I04, the normally closed contacts lie of the contactor II, and the conductors II and 44 to the conductor LI. The energizing circuit for the winding 2210 01' the contactor 22 is from the energized segment 4| to the segment I! and through the conductor 99, the now closed contacts I341 of the contactor II, a conductor "II, the conductor II, the winding 2210, and the conductor II to the conductor Li.

. The closure oi the contactor 36 in response to energization of its operating winding 36w completes the circuit to the operating winding IIw oi the brake 31 which is thereby released.

With the contactors Ill and I3 closed, two terminals of the primary winding of the motor I4 are connected together and the interconnected terminals connected to the conductor L2 and the third terminal of the primary winding of the motor i4 is connected to the conductor Ll. The resistance sections 0 and d remain in the secondary circuit, the sections a and b being short circuited by the contacts of the contactor 22. If a non-overhauling load is connected to the hoist, no movement will occur since the singlephase supplied to the primary winding will cause no torque. If, however, an overhauling load is connected to the hoist, it moves downward in opposition to the braking torque oi the singlephase connection and reaches a stable speed either above or below the synchronous speed of the induction motor. If it is desired to maintain the average speed 01' the load at a still lower value, the master switch may be repeatedly moved between the on position and the first lowering position, thus intermittently applying :xe mechanical brake 31 which tends to stop tir load entirely, and intermittently applying single-phase braking, which tends to permit a slow movement of the load.

Movement of the master switch to the second position in the lowering direction causes the deenergization of the operating winding 22w 0! the contactor 22 which thereupon opens to insert additional resistance sections a and b into the rotor circuit. As a result. the braking torque is reduced, and the lowering speeds of all overhauling loads increase. The braking torque has been reduced from that indicated by the curve VI oi Fig. 5 to that indicated by the curve Vi. Examination of these curves shows that the braking torque is eflective at speeds above synchronism as well as at speeds below synchronism, gradually increasing as the motor speed increases. This gradual increase in braking torque at speeds above synchronism permits very definite speed control since any tendency tor the motor to go faster is immediately opposed by an increased braking torque.

Movement of the master switch to the third position in the lowering direction causes the deenergization of the operating winding i310 oi the contactor I3 and energization of the operating windings llw and 3220 of the contactors l2 and 32, respectively. The contactor Ill remains closed. The energizing circuit for the winding I220 is from the energized segment 4| to the segment 49 and through a conductor II, the winding i210, a conductor 42, and the conductor 84 to the conductor L3. The energizing circuit for the winding 32w of the contactor I2 is from the energized segment 4i to the segment 58 and through a conductor IS, the winding 32w, a conductor 44 and the conductor II to the conductor Ll. 0pening of the contactor i3 removes the single-phase braking from the motor I4 and the closing or the contactor l2 applies three-phase power and causes a downward torque to be exerted. Closure of the contactor II connects the relay circuit 34 across two or the slip-rings III which, at speeds below of synchronous speed, causes the winding 38w to be energized to cause operation of the relay I3.

Since all of the external resistance rl, T2 and 1'! is connected in the rotor circuit of the motor 14, an overhauling load causes the motor to accelerate toward synchronous speed. As soon as the speed or the motor reaches a predetermined speed, for example, 50% or synchronous speed, or if the speed is already above 50% of synchronism, the relay 33 operates to energize the operating winding 26w of the contactor 24%. The energizing circuit for the winding 24w is from the energized segment ill to the segment ti and through a conductor Hit, the now closed contacts 33c of the relay 33, a conductor Hit, the winding 24w, and the conductor ii to the conductor Li. The closing of the conductor 24 in re= sponse to the energization of its operating wind= ing 26w short circuits all of the external rotor resistances rt, T22, and r3. Overhauling loads under this condition move downward at speeds slightly above synchronous speed and light loads move downward at speeds slightly below synchro nism.

If the speed of the motor in the third position lowering is such that the contactor 26 has not closed, the movement or" the master switch to the fourth position causes an increase in speed due to the closure of the contractors 22 i, 22, 23, and 2d and consequent short circuiting of all of the secondary resistance. It the contactor 2d already has closed by virtue of the operation of the relay the movement of the master switch to the fourth position does not alter the speed of the load, but merely causes closure of the contactors 2t, 22 and The energizing circuit for the contactor ti is from the energized segment it to the segment 55 and through a conductor 6%, the conductor ill, the winding it w, and the conductor ii to the conductor Li. The energizing circuit for the winding 2220 of the contactor 22 is from the energized segment ti to the segment tit and through a conductor iti, the now closed contacts'2ia of the contactor iii, the conductor it, the winding 22w and the conductor ll to-the conductor Li. The energizing circuit for the winding 2310 of the contactor 23 is from the energized segment M to the segment 53 and through a con ductor Hi8, the now closed contacts 22a of the contactor 22, the conductor I6, the winding 23w and the conductor 'II to the conductor Ll. The energizing circuit for the winding 24w which is independent of the why 33 is from the energized segment H to the segment 52 and through a conductor I09, the now closed contacts 230. of the contactor 23, the conductor l8, the winding 24w and the conductor II to the conductor Ll.

If the master switch is now moved to the fifth position in the lowering direction, a further increase in the speed of overhauling loads takes place due to the fact that the contactors 23 and 24 are opened as a result of the deenergization of their operating windings 2310 and 24w to reinsert the resistance sections 0 and d into the rotor circuit.

If the speed of the motor should reach 150% of its synchronous speed with the master switch 40 in the fifth position, the relay circuit 30 causes operation of the relay 33, resulting in closure oi. the contacts 33a and 33b. The closure of the contacts 33a at this time completes a circuit to the winding 2310 which is traceable from the energized segment 4| to the segment 55 and through the conductors I08 and N0, the contacts 33a, a conductor HI, the-conductor I08, the closed contacts 22a, the conductor 16, the winding 2310 and the conductor II to the conductor Li. The contactor 23, in response to the energization of its operating winding 23w, closes its contacts to again short circuit resistance section c to reduce the speed of the motor and maintain the speed below 150% of synchronous speed.

If, with the master switch in the fifth position in the lowering direction, the load is not such as to cause speeds in excess of 150% synchronous speed, further increases in speed may be ob talned by moving the master switch it to the sixth position, causing deenergization of the winding 22w'and consequent opening of the contactor 22. The opening of the contactor 22 reinserts resistance sections is into the rotor circuit. If the speed at the sixth position increases above 150% of synchronous speed, the relay circuit at causes energization of the winding 3310 and consequent closure or the contacts it?) of the relay 33. Closure of the contacts filth cornpletes a circuit to the winding 22w from the now energized conductor lid through the contacts 33? to a conductor M2, the conductor till, the closed contacts tic of the cohtactor '20, the con ductor 713, the winding 2220 and the conductor it to the conductor Li. Energization of the op-= crating winding 22w causes closure of the con-= tactor 22, to again short circuit the resistance sections a and b, resulting in a decrease in speed. As soon as the contacts 23a of; the contactor close, contactor 23 will also close, due to energize,

tion of its winding over a circuit through the contacts 33a and previously traced in connection with the fifth position lowering. Contactor 253, in response to the energizatlon of its operating winding 23w, closes its contacts to again short circuit the resistance section c to prevent the motor it from going above 150% of its synchronous speed.

It is clear from the above description that in the case of overhauling loads, the master switch may be moved from positions one to six to cause gradual increases in speed, provided that a maximum predetermined speed is not exceeded, and that movement of the master switch from the sixth position back toward the oil position causes gradual decreases in speed, provided that the maximum predetermined speed has not been reached.

In the described system, when the master switch is in the first or second position of lowering, no downward torque is exerted. In the third position a downward torque is exerted, with all of the external rotor resistance in the circuit. Since non-overhauling loads require but slight downward torque, if the master switch is left in the third position, the speed generally immedi-- ately approaches synchronism. Slower average speeds may be maintained by repeatedly moving the master switch between the second and third positions. With the motor exerting but little torque, increasing or decreasing the secondary resistance has no appreciable eflect on the speed. The important thing is that by moving the master switch away from the of! position to the third position, an increase in speed takes place, for both overhauling andnon-overhauling loads, and by moving the master switch from the third position toward the off position, a decrease in tilt speed takes place for both overhauling and nonoverhauling loads.

. Although the description has assumed that the and the described sequence will take place due 7 brake 31 to set.

to the interlocking contacts in a manner well unders ood.

Although single-phase braking has been shown as effective at only two master switch positions, further speed control for overhauling loads can be obtained by providing more positions of single-phase excitation, each with a diil'erent amount of external secondary resistance to predetermine the braking torque. Fig. 5 shows that a reduction in the amount of secondary resistance with single-phase braking connections established causes an increase in braking torque up to a certain point beyond which further reduction causes a decrease in braking torque at speeds below synchronism, and an increase in braking torque at speeds above synchronism. The proper value of secondary resistance to use for any particular master switch position can be found easily by experiment. For example, one of the motors in the experiments which resulted in the data used to plot the curves of Fig. 5, required 3.4 ohms of external resistance to give a braking characteristic corresponding to curve V3 and 7.? ohms to give a braking characteristic corresponding to curve VI.

Although in Fig. 4 the mechanical brake 31 is operated by direct current obtained from the rectifier 35, other types of mechanical brakes can be used, but, since there is no necessity of having a source of direct current available, these other types of brakes may be operated most conveniently by alternating current. All alternating current brakes have the disadvantage of being slowsetting, that is, when the current through the operating winding is interrupted, a few seconds elapse before the brake shoes contact the brake drum with suiflcient pressure to cause a braking action. Reference to Fig. 4 shows that when the master switch 40 is moved from the first position in the lowering direction to the off-position, the single-phase braking circuits and the circuit to the winding 31w of the brake 31 are interrupted simultaneously. As a result there is an interval during which no braking force is efiective on the motor I4, the duration of the non-braking interval depending upon the time it takesfor the In direct current crane control this condition of non-braking is not present due to the fact that the dynamic braking circuit is maintained closed while the master switch is In the off-position. However, due to power losses, it is not desirable in the case of single-phase braking to have the braking circuits efl'ective at all times while the master switch is in the oil-position.

The modification shown in Fig. 6 illustrates how the controller of Fig. 4 may be changed so that the single phase braking torque is effective during the entire interval of switching from the first position lowering to the off-position and until the mechanical brake sets.

In Fig. 6, elements similar to those in Figs. 1 and 4 are referred to by the same numerals. A portion only of a master switch I00 is illustrated and includes contact segments 4i and I I5 to I22, it being understood that the remaining portion of the master switch I40 and the connections to the control circuits for the motor I4 may be the same as the corresponding portion of the master switch 40 and corresponding connections of Fig. 4. The contact segments II! and H8 control the energlzation of a winding I3Iiw of a contactor I36 which in turn controls the operation of the mechanical brake I31. The brake I31 is diagrammatically Illustrated as an alternating current brake which sets to stop the motor I4 upon interruption of the power supply by the contactor I26. The contact segments III and III control the energization of the operating winding I0w of the contactor III, and'the contact segments II! and I20 control the energization of the operating windings I3w and 22w respectively of the conshown as a dashpot I23). The winding I23w is energized in all lowering positions through the contact segment I22. The windings I0w, I3w and 22w may be energized independently of the contact segments III, H8, H9 and I20 through the contact segment I2I and the contacts I23a, I23b and i230, respectively, of relay I23.

Only one position of single phase braking is provided on the master switch I40, but, when it and the associated circuits are substituted for the corresponding portion of the master switch 40 and associated circuits of Fig. 4, the operation otherwise is the same as that of Fig. 4 except for the changes effected by the inclusion of the time delay relay I23. Movement of the master switch I40 from the oil-position to the first position in the lowering direction causes energization of the operating windings I36w, I021), I311) and 22w. The mechanical brake I31 is released upon energizetion of the winding I36w and consequent closure of the contactor I38 and the connections for single phase braking are set up by the operation of the contactors I 0, I3 and 22 in the same man ner as in the case of the use of the master switch 40. Also, in the first lowering position, the winding I23w oi the relay I23 is energized and the contacts I23a, I23b and I230 are instantly closed. Movement of the master switch I40 to the second lowering position applies downward power to the motor I4, and movement to subsequent positions in the lowering direction varies the secondary resistance of the motor I4 in the same manner as described in connection with the master switch 40. In the second and subsequent lowering position the contactor I3 is open and the energization of the winding 2220 of the contactor 22 is controlled by the contact segments 46 and 54 and the interlock contacts 2Ia and 2Ib of the accelerating contactor 2I. The winding I 23w remains energized in all lowering positions through the contact segment I22, but the closed contacts of the relay I23 are of no effect in positions other than the first lowering position and the off-position since the contact segment I2I is in contact with a terminal 58 only in the off-position and the first lowering position of the master switch I40. k

Assuming that the load is moving downward, and it is desired to stop its movement, the master switch may be returned to the 0ff-position. When in the off-position the winding I23w is deenergized, but the contacts of the relay I23 remain closed, due to the time delay action of the dash pot I23), and complete a circuit through the segment I2I to the windings I0w, I3w and 22w. As a result the single phase braking connections remain effective to cause a braking action during the setting interval of the mechanical brake I31 which begins to set due to the deenergization of the winding IIGw as a result of the movement of the master switch I 40 to the elf-position. The

time delay period of the relay I23 is adjusted so that the contacts I 23o, i231; and l23c open as soon as the mechanical brake ltl operates to hold the losol, and, since the single phase braking connections remain until the relay its opens its contacts, there is no lack of braking? force during the setting interval, of the mechanical brake ill.

Having thus described my invention, I claim: 1. The combination with. a three-phase dynamo electric machine having a primary winch ing which is o seconclory winding connected in a closed circuit, a source of three-phase power, means 0 stable for conrlectimg two lorarlcilcs of is said primary winding in parallel with e ch other and both in series with the third branch, the series parallel combination "coins; conuectable across one chose of saitl source, of a resistor in saiol closeol circuit, moons to remove the re sister from a closed circuit while maintaining said circuit closeol to permit dynamo electric machine to operate a motor, and means for connecting said primary winding in star to said three phase source oi power to cause ro-= tstioh oi dynamo electric machine as a mo tor, whereby the oy'ocmo electric machine oper stes efficiently cormectcd ss motor throughout an unbroken from core speed to anti in eluding oveusmochronous speedsami operates emcien-tly es s broke throughout an imhrohcn range from soeetls above synchronous sip-coals to zero speeci.

2. In system for a polyphasc inductlon motor having a primary circuit o Soc onciary circuit iocluoinga secondary winding and on soiusta-lole resistcuce and being suhiect to overhauling loads, moons to connect the primsmr circuit to a polyphase source of sup-ply to cause a motor torque for assisting: rotation of said moso tor in the direction or the overl'rauling" torque created by salcl overhauling loath, means short circuit a portion of sold primary circuit anal to connect st least a portion of the primary circuit to a single chose source of current to cause a worm-ring torque opposite to the torque created by sold overhauling load, anal means operable when the motor is connectool to produce e motor torque to vary the adjustable resistonce to core trol the amount of saicl. motor torque and oper able when the motor is connected to produce a braking torque to vary the adjustable resistance to control the amount of saiol braking torque.

3. A, control system for a wound rotor inouction motor subject to overhauling loads and having a star=connecteol primary winding arranged for connection to a three-phase source of power having three output conductors, a me-ster control element movable to several sequential motor control oositlorrs, movement of saicl element in one direction causing it to attain each of said positions in e, predetermined sequence, means operable when sairl element is in the first position to maintain said primary winding EMS- connectecl from said source, moons operative when said master control element is in the next sequential position to connect two notuneutrai terminals of said primary winding to one of said conductors and to connect the other non neutral terminal of solo primary winding to another of said con' uotors to produce a braking torque on said rotor, and means operative when'saicl master control element is in the next sequential position to connect said primary winding to said ll three-phase source in a manner to cause a torque to be exerted on said rotor to assist the torque of said overhauling load.

4. A control system for an induction motor subject to overhauling loads and having a polyphase primary circuit and a. secondary circuit, and arranged for connection to a polyphase source of power, an adjustable resistance for the secondary circuit, a master control element movable to several sequential motor control positions, movement of said element in one direction causing it to attain each of said positions in a predetermined sequence, an electrically operable mechanical brake, means operative when saicl master control element is in the first sequential position to apply said mechanical brake to hold said load at standstill, means operative when said master control element is in the next Sequential position to release said mechanical brake, to short circuit a portion of said primary circuit, to energize at least a portion of said primary circuit with single phase current from said three-phase source of power, and to associate saiol adjustable resistance with the secondary winding, whereby a braking torque is pro-= duceol, and means operative when said master control element is in the next sequential position to connect said primary winding in star to said three-phase source lira manner to cause a torque to be exerted to assist the torque of said overhauling load.

5. A control system tor a polyphase wound r0-= tor induction motor subjected to overhauling loads and. arranged for connection to a source of current supply for said motor, means to energize the motor with polyphase current from the source of supply, external resistance con necterl in the rotor circuit of saicl motor, a mas ter control element, switching means responsive to movements or the master control element in one direction to short circuit said resistance in steps to gradually increase the speed of said motor in a direction opposite to the torque exerted by seicl overhauling loacl, and means operable to connect the motor to single-phase source of cur rent in a manner to produce a braking torque, said switching means being responsive to movements of the master control element in another direction to short circuit saicl resistance in steps and thereby gradually decrease the braking torque of said motor opposing the torque exerted by saiol overhauling loaci, whereby the speed of operation of said. motor in each direction hears a definite relation to the extent and direction oi movement of said master control element.

6. The combination with a polyphase wound rotor induction motor drivingly connected to an overhauling load and arranged for connection to a polyphase source of current, manually operated means for varying the rotor resistance to control the speed of said motor, of means ro sporrsive to the electrical condition of the rotor circuit of said motor when the motor is oper' atively connected to a polyphase source of cur" rent for varying the rotor resistance to pre= vent overspeeding oi the motor due to said overhauling load throughout a predetermined range, and means for connecting the motor to a, single phase source of current in a manner to produce a braking torque on the rotor within a. predetermined range of speeds at least partly beyond said first range.

7. A control systemv fcra'polyphase induction motor connected to an overhauling load and hav= ing a primary and a secondary circuit and arranged to be connected to a source or polyphase power and comprising an adjustable resistance for the secondary circuit, a master control element movable to several different control positions, means operative when said element is in a first position to set up connections for single phase braking of said motor, means operative when said element is in a second position to connect said motor to said source to cause said motor to exert a torque in the direction of the torque created by said overhauling load and to connect said secondary resistance in said secondary circuit, means responsive to the electrical condition of the secondary circuit to short circuit said secondary resistance it the speed oi. said motor exceeds a predetermined minimum when said element is in said second position, means re sponsive to movement of said element to subsequent positions to selectively control the amount of said secondary resistance connected in the sec ondary circuit, said which is responsive to the electrical condition of said secondary circuit being operable to short circuit portions oi. said secondary resistance when. the speed of the motor exceeds a second predetermined minimum while said element is in said subsequent positions.

8. A control system for on induction motor drivingly connected to an overhauling load capable 0! driving said motor at s mods greater than. the synchronous speed of said motor, comprlsiuc means operable to slow down said motor from speeds greater than sold synchronous speed while the motor is being clrivorzz by said cverlmul in: load, and means rcswnslvc to the electrical condition of the secondary circuit oi said motor when said motor is being driven at a predctermined speed greater than soul synchronous speed to cause operation or the first mentioned means to slow down said motor.

9. The combination with a polyphase wound rotor induction motor drlvlngly connected to an overhauling load capable oi driving said motor at speeds greater than the synchronous speed of said motor, a resistor normally included in the rotor circuit of said motor, and means operable to exclude said resistor from said rotor circuit, of means responsive to the electrical condition of the rotor circuit when said motor is operating at a predetermined speed above said synchronous speed to cause operation 01' said first-named means to exclude said resistor from the rotor circuit.

10. In a control system for a poiyphase induction motor drivingly connected to an overhaulin: load, a resistor normally included in the rotor circuit, manual means to connect said motor to a lource or power in a manner to produce a motor torque to assist said overhauling load, relay mean; responsive to the electrical condition of the rotor circuit at a predetermined speed below synchronous speed to exclude said resistor irom laid rotor circuit, manual means operable to include a portion of said resistor in the rotor circuit regardless of the operation of said relay means, and said relay means being responsive to the electrical condition 01' the rotor circuit at a predetermined speed above synchronous speed to at!!! exclude said resistor from said rotor circuit.

11. The combination with a polyphase wound rotor induction motor drivingly connected to an overhauling load and arranged for connection to 5 polyphale source of current and manual operating means for varying the rotor resistance to control the speed or said motor, of means responsive to the electrical condition of the rotor circuit or said motor when the motor is open atively connected to a polyphase source or current and rotating at speeds above synchronism for automatically varying the rotor resistance to prevent over-speeding oi the motor due to said overhauling load, and manual means operable while the motor is rotating at speeds above synchronism for connecting the motor to a single phase source oi! current in a manner to produce a braking torque.

12, The method of electrically braking an induction motor having a polyphase primary winding' and a secondary circuit including a rotor winding and. an external resistor, while said motor is rotating at speeds throughout a pre-- determined range above synchronous speed, and comprising short circuiting e. portion or the primary winding and, While said portion iii shortcircuited, connecting at least a portion of the primary Winding to a source of single-whose power and, while maintaining said short circuit curl said single phase connection, closing the secondary circuit through said external resistor all While said motor is so rotating.

13. The combination with c. polyphase wound rotor induction motor drivingly connected to an overhauling it. d and arranged for connection to a polyphuce urce of current, manually operouted means varying the rotor resistance to control the *eti of said motor and moons responsive "to p motor is or til/bible source cl current to once to pro to or ruined t to u cinclc ii s to "produce tucking torque on he rotor within a predetermined range oi speeds least partly beyond said first range,

i to polyphesc varying rotor resist t o'vercpeedirm ct cling" load thrc meant; for cm. i

14. A control system for a mlyphuse induction motor connected to an overhauling load and himlng a primary and a secondary circuit and arranged to be connected to a source of polyphase power, and comprising an adjustable resistance for the secondary circuit, a master control element movable to several diflerent control positions, means operative when said element is in a first position to set up connections for single phase braking of said motor, means operative when said element is in a second position to connect said motor to said source to cause said motor to exert a torque in the direction of the torque created by said overhauling load and to connect said secondary resistance in said secondary circuit, means responsive to the speed of said motor to short circuit said secondary resistance ii the speed 01 said motor exceeds a predetermined minimum when said element is in said second position, and means responsive to movement of said element to subsequent positions to selectively control the amount of said secondary resistance connected in the secondary circuit, said means which is responsive to the speed 01 said motor being operable to short circuit portions of said secondary ruistance when the speed 01 the motor exceeds a second predetermined minimum while said element is in said subsequent positions.

15. In a control system for a polyphase induction motor drivingly connected to an overhauling load, a resistor normally included in the rotor circuit, manual means to connect said motor to a source of power in a manner to produce a motor torque to assist said overhauling load, relay means responsive to the speed of said motor at a predetermined speed below synchronous speed to exclude said resistor from said rotor circuit, and manual means operable to include a portion of said resistor in the rotor circuit regardless of the operation of said relay means, said relay means being responsive to the speed of said motor at a predetermined speed above synchronOlls speed to again exclude said resistor from said rotor circuit.

16. In a braking system for a polyphase induction motor having a primary circuit and secondary circuit and drivingly connected to an overhauling load capable of driving said motor at speeds above svnchronism, switching means operable for short circuiting a portion of said primary circuit and for connecting at least a portion of said primary circuit for energization by a single phase current, a secondary resistor, switch means operable to include a predetermined portion of said resistor in said secondary circuit, and means operable while said motor is being driven at speeds above synchronism for effecting operation 01 said switching means and said switch means, thereby causing a torque to be produced in the motor which is opposite to the torque of said overhauling load.

17. The combination with a three-phase induction motor having a star connected primary winding and a secondary circuit and means for driving said motor at speeds above its induction motor speed, switching means operable for connecting two branches of said star connected primary winding in parallel with each other and both in series with the third branch across a source of single phase current, a secondary resistor, means operable to include a predetermined portion of said resistor in said secondary circuit, and means operable while said motor is being driven at speeds above synchronism for effecting operation of said switching means and said last named means, thereby causing a torque to be produced in the motor which is opposite to the torque of said overhauling load.

18. The combination with a polyphase wound rotor induction motor drivingly connected to an overhauling load and arranged for connection to a polyphase source of current and manual operating means for varying the rotor resistance to control the speed of said motor, of means responsive to the speed of said motor when the motor is operatively connected to a polyphase source of current and rotating at speeds above synchronism for automatically varying the r'otor resistance to prevent over-speeding oi the motor due to said overhauling load, and manual means operable while the motor is rotating at speeds above synchronism for connecting the motor for energization by a single phase current in a manner to produce a braking torque.

HARRY L. wmcox.

CERTIFICATE O CORRECTION. Patent No. 2,255 501. March LL, 19in.

HARRY L.. wILcox.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5 first column, line 51, and page 5, second colnmn, line 27, for "12" read --L2-;

page 9, first column, line 12, claim 1, fcr the words "which is a secondary winding connected" read --and a secondary winding which is connected;

and that the said Letters Patent should be read with this correction therein that the same may conform to'the record of thecase in the Patent Office.

Signed and sealed this 15th day of May, A. D. 1914.1.

Henry Van Arsdale, (S Acting Commissioner of Patents.

DISCLAIMER 2,233,501.Harry L. Wilcox, Cleveland, Ohio. BRAKING SYSYTEM FOR INDUCTION MOTORS. Patent dated Mar. 4, 1941. Disclaimer filed Sept. 23, 1947, by

the assignee, The Electric Controller &: Manufacturing Company. Hereby enters this disclaimer to claims 1, 12, 16, and 17 in said specification.

[Ojficial Gazette October 28, 1947.] 

